The present invention relates to charged particle beam apparatuses for measuring the dimensions of the microscopic pattern of a specimen, and in particular, relates to a charged particle beam apparatus that is equipped with a mechanism and a control method for controlling the gradient of a primary charged particle beam.
A scanning electron microscope (SEM), which is a type of a charged particle beam apparatus, accelerates primary electrons emitted from an electron source, converges the primary electrons using an electrostatic lens or an electromagnetic lens, and irradiates the primary electrons to the surface of a specimen. Secondary electrons are emitted from the specimen due to the irradiation of the primary electrons (in some cases, among the secondary electrons, electrons with low energy are referred to as “true” secondary electrons, and electrons with high energy are referred to as reflected electrons). Because the generation amount of secondary electrons becomes large at the edge portion of a pattern of the specimen, by scanning the surface of the specimen with primary electrons that are electromagnetically deflected, and by detecting secondary electrons emitted from some irradiation points of the surface of the specimen, a scanning electron microscope image (SEM image) that reflect the shape and composition distribution of the specimen is obtained. In addition, it is also possible to form an absorbed current image by detecting electrons absorbed in the specimen. Because the microscopic pattern of a specimen can be observed by locally converging primary electrons using an SEM in such a way, the SEM has been used for measuring the dimensions of microscopic patterns in semiconductor manufacturing processes.
In the case of using an SEM, if primary electrons are not deflected, the incident angles of the primary electrons into a specimen are almost perpendicular in many cases. On the other hand, if primary electrons are deflected, the incident angles of the primary electrons vary because the trajectories of the primary electrons change. If the incident angles vary, even if plural images of the same specimen pattern are obtained, the images of the plural images are viewed differently and the measurement values are also different from each other.
In recent years, the measurement of the dimensions of the bottom having three-dimensional structure has been increasingly needed in the measurement of the dimensions of semiconductor circuit patterns. In particular, the observation or dimension measurement of the bottom parts of deep grooves and deep holes having an aspect ratio of nearly 50 has been widely required. Furthermore, there is also a need for the measurement of a misalignment between the center of the upper surface of a deep groove or a deep hole and the center position of the bottom surface of the deep groove or the deep hole. In the measurement of a shape having such a high aspect ratio, the incident angles of primary electrons have a large effect on the measurement results. Therefore, in order to realize the high-accuracy measurement of a three-dimensional structure of a specimen typified by a deep groove or a deep hole, it is necessary to inject primary electrons perpendicularly into the specimen even in the case where the primary electrons are deflected.
To cope with this problem, a method in which the incident angles of primary electrons are set equal in a deflection region is disclosed in Japanese Unexamined Patent Application Publication 2007-187538, and a method in which the incident angles are controlled using a correction term is disclosed in Japanese Unexamined Patent Application Publication 2012-234754.